Of recent rolling bearings, many bearings such as those for rocker arms used for opening/closing an intake valve or exhaust valve of an engine, for example, are applied to those uses under high-speed heavy-load conditions in spite of the fact that these bearings are full-type rolling bearings. In particular, regarding a full-type rolling bearing without cage, skew is likely to occur for the reasons that rollers interfere with each other and roller positions are not smoothly controlled. Further, if a lubricating oil is not appropriately supplied into the bearing, an inadequate lubricating condition is likely to occur. As a result, heat is generated from sliding or the surface pressure is locally increased. Thus, in spite of a large load-carrying capacity as calculated, surface damages (peeling, smearing, surface-originated peeling) and inside-originated peeling are likely to occur.
Like a bearing for a rocker arm used for opening/closing an intake valve or exhaust valve of an engine, for the bearing having an outer ring whose outer circumference is in rolling contact with a cam, many of improvements have been made chiefly for the purpose of improving the outer circumference of the outer ring. For example, compression residual stress induced by such a process as shot peening and increased hardness induced by high-concentration carbonitriding (process-induced effects) have been used to extend the life, chiefly for improving the circumference of the outer ring that is in rolling contact with the cam.
Some techniques that have been known take the following measures.
(1) In order to improve the rolling fatigue life, shot peening is performed on the race of a bearing component, and a reinforced layer, a retained-austenite-containing layer and a quench-hardened layer are provided in the bearing component from the surface side to the inner side in this order (Japanese Patent Laying-Open No. 2-168022 (Patent Document 1)).
(2) A technique that efficiently adjusts the size and area ratio of carbide in a martensite structure, the amount of retained austenite and the hardness, by performing shot peening (Japanese Patent Laying-Open No. 2001-065576 (Patent Document 2)).
(3) In order to improve the rolling fatigue life, the residual-compression-stress peak level and distribution resultant from shot peening are made equal to the maximum shearing stress and operational depth that is exerted in use (Japanese Patent Laying-Open No. 3-199716 (Patent Document 3)).
(4) A control method for a carburized bearing, applying shot peening for prolonging the life, and providing a final-surface-finished surface so that the combination of residual compression stress σ (MPa) and retained austenite γ (%) satisfies: 0.001σ+0.3γ≧1.0.
(5) A cam follower device (bearing for rocker arm) has a cam-follower outer ring whose outer circumference is equivalent in hardness to an associated cam and whose inner circumference is higher in hardness than the outer circumference (Japanese Utility Model Laying-Open No. 3-119508 (Patent Document 4)).
(6) In a component that is in rolling contact or rolling slide contact with another opposite component, a surface layer with a depth of 0 to 50 μm from the surface has a maximum compression stress of 50 to 110 kgf/mm2, a hardness of HV 830 to 960, retained austenite of at least 7%, and an average surface roughness of at most 25 μm, which are achieved through shot peening (Japanese Patent No. 3125434 (Patent Document 5)).
Although improvements for prolonging the rolling life of an outer-ring shaft corresponding to the inner ring, rollers and the whole bearing have been relatively few, some improvements have been made as shown below in terms of material properties for providing heat resistance and microstructure stability as well as increased hardness by carbonitriding, and thereby prolonging the life of the bearing.
(7) For a cam follower bearing (bearing for rocker arm) of an engine valve mechanism, a calculated life of the bearing, at a rated engine rpm, of 1000 hours or longer is achieved (Japanese Patent Laying-Open No. 2000-038907 (Patent Document 6)).
(8) In order to achieve a bearing shaft for a cam follower device (bearing for rocker arm) having the carbide ratio=10 to 25%, the ratio of decomposed austenite content to initial retained austenite content= 1/10 to 3/10, the end-face hardness=HV 830 to 960, and average surface roughness of 25 μm or less, a bearing steel is carbonitrided and hard shot peened (Japanese Patent Laying-Open No. 10-047334 (Patent Document 7)).
(9) A solid lubricant film of high polymer for example is formed on a cam follower shaft (bearing component for rocker arm) for improving wear resistance of the shaft (Japanese Patent Laying-Open No. 10-103339 (Patent Document 8)).
(10) A cam follower shaft (bearing component for rocker arm) is made of a tool steel for example and is ion-nitrided or ion-plated at a temperature lower than a tempering temperature so as to have a high hardness (Japanese Patent Laying-Open No. 10-110720 (Patent Document 9)).
(11) A cam follower bearing for an engine valve mechanism (bearing for rocker arm) having a bending stress of 150 MPa or less with respect to the shaft (Japanese Patent Laying-Open No. 2000-038906 (Patent Document 10)).
(12) A cam follower for an engine valve mechanism (rolling bearing for valve mechanism) that has a phosphate film which is excellent in lubricating-oil retention and provided on a rolling surface of a bearing component (Japanese Patent Laying-Open No. 2002-031212 (Patent Document 11)).
(13) A cam follower for an engine valve mechanism (bearing for valve mechanism) that has a crowning in a region of a shaft where rollers roll (Japanese Utility Model Laying-Open No. 63-185917 (Patent Document 12)).
(14) A carburized shaft having a rolling surface layer which is high-concentration carburized or carbonitrided with a carbon concentration of 1.2% to 1.7% and having an internal hardness of HV 300 (Japanese Patent Laying-Open No. 2002-194438 (Patent Document 13)).
There is another problem regarding the rocker arm. In such a case where both ends of an outer-ring shaft are caulked to be fixed to an outer-ring support member, although a rolling surface of the outer-ring shaft should have a high hardness, the ends thereof should be soft enough to be caulked. Further, after the shaft ends are caulked to be fixed, the strength (hardness) should be high enough for preventing loosening in use. The following document discloses caulking of both ends of an outer-ring shaft of a roller rocker.
(15) The outer surface of an outer-ring shaft is uniformly induction-hardened and then tempered, and thereafter only the ends of the shaft are high-frequency annealed and accordingly softened (Japanese Patent Laying-Open No. 05-179350 (Patent Document 14)).
In recent years, environmental issues have become salient. For automobiles as well, enhancement of fuel economy is legally mandated and strongly called for. Accordingly, components constituting an engine are also required to have a lighter weight and compactness. Therefore, a rocker arm used for opening/closing an intake valve or exhaust valve of an engine is also required to be lightweight and compact and naturally, a bearing for the rocker arm is also required to be lightweight and compact.
As a method of reducing the weight of a bearing, U.S. Pat. No. 4,727,832 (Patent Document 15) discloses a method employing an inner ring that is a hollow cylinder in shape (pipe). The bearing may be made compact by being downsized. However, as for the downsizing of the bearing, because of limitations due to the shape of the cam and the lift amount in valve opening/closing, the size cannot be decreased simply and thus downsizing is chiefly done in the direction of the width (shaft). Therefore, the method for making the bearing lightweight and compact is chiefly achieved by providing an inner ring of hollow cylinder in shape and downsizing in the direction of the width.
Here, in making the bearing lightweight and compact, some issues have to be considered. They are deterioration in crack fatigue strength of the outer ring or inner ring and shortening of the rolling fatigue life of the roller or inner ring.
The applicant has been successful in improving, through improvements in heat treatment technique, crack fatigue strength and rolling fatigue life for an outer ring, rollers and an inner ring (entirely hardened inner ring) that undergoes press-fitting, pinning or the like (except for caulking) to be fixed to a rocker arm. Therefore, for an inner ring that is caulked to be fixed to a rocker arm, it is necessary to improve crack fatigue strength and rolling fatigue life.
The applicant has also been successful in improving rolling fatigue life of an inner ring that is caulked to be fixed to a rocker arm. Thus, a remaining goal is to improve crack fatigue strength of an inner ring having a hollow cylindrical shape.
Conventionally, for an inner ring caulked to be fixed to a rocker arm, a hollow cylindrical inner ring is not used because process steps increase and instead a solid cylindrical inner ring is employed. In order to allow an end surface to be caulked, only a raceway surface for rollers that are rolling elements is surface-hardened by high-frequency heat treatment. This is disclosed for example in Japanese Patent Laying-Open No. 62-007908 (Patent Document 16) and Japanese Patent Laying-Open No. 5-321616 (Patent Document 17).
Similarly, in the case where a hollow cylindrical inner ring is used, in order to allow an end surface to be caulked, conventionally only a raceway surface for rollers that are rolling elements is surface-hardened through high-frequency heat treatment. This is disclosed for example in Japanese Patent Laying-Open No. 3-031503 (Patent Document 18) and Japanese Patent Laying-Open No. 2000-038906 (Patent Document 10).
In particular, a hollow cylindrical inner ring has a larger bending stress due to its shape, as compared with a solid cylindrical inner ring. Therefore, there is a method providing the bending stress of 15 kgf/mm2 or less to prevent breakage (see Japanese Patent Laying-Open No. 2000-038906). However, this method imposes limitations on the shape, and thus hinders the bearing from being made lightweight and compact.
Further, for an inner ring having a simple hollow cylindrical shape, since the strength is insufficient due to the above-described reason, a reinforcing material may be attached to the inner circumferential portion (see Japanese Utility Model Laying-Open No. 3-063703 (Patent Document 19)). This method, however, is accompanied by increased process steps and complexity, disadvantageously resulting in a higher price of the bearing.
Patent Document 1: Japanese Patent Laying-Open No. 2-168022
Patent Document 2: Japanese Patent Laying-Open No. 2001-065576
Patent Document 3: Japanese Patent Laying-Open No. 3-199716
Patent Document 4: Japanese Utility Model Laying-Open No. 3-119508
Patent Document 5: Japanese Patent No. 3125434
Patent Document 6: Japanese Patent Laying-Open No. 2000-038907
Patent Document 7: Japanese Patent Laying-Open No. 10-47334
Patent Document 8: Japanese Patent Laying-Open No. 10-103339
Patent Document 9: Japanese Patent Laying-Open No. 10-110720
Patent Document 10: Japanese Patent Laying-Open No. 2000-038906
Patent Document 11: Japanese Patent Laying-Open No. 2002-031212
Patent Document 12: Japanese Utility Model Laying-Open No. 63-185917
Patent Document 13: Japanese Patent Laying-Open No. 2002-194438
Patent Document 14: Japanese Patent Laying-Open No. 5-179350
Patent Document 15: U.S. Pat. No. 4,727,832, specification
Patent Document 16: Japanese Patent Laying-Open No. 62-007908
Patent Document 17: Japanese Patent Laying-Open No. 5-321616
Patent Document 18: Japanese Patent Laying-Open No. 3-031503
Patent Document 19: Japanese Utility Model Laying-Open No. 3-063703